What the other posters have said is absolutely true. However, the issue often of interest to users is 'Will the iPad eat up my data allowance if I leave it switched on and WiFi is available'? Technically, of course, the answer is 'No', but the problem arises, for example, when your iPad is connected by WiFi to, say, your ADSL broadband modem and you leave your 3G switched on and WiFi stops working, for some reason (kids knocked the plug out from its power supply). The iPad will then swap to 3G and *it doesn't alert you*. As a result you can inadvertently be using 3G without realising it.
Tim
Scotland